Controlled environment enclosure and mechanical interface

ABSTRACT

A system for vacuum-processing objects such as electronic integrated circuit wafers comprises (a) a carrier for transporting the wafers under vacuum in a cassette, the cassette being supported on a movable wall that serves as a bottom cover member of the carrier; and (b) a processing machine having a transfer chamber that is also maintained under vacuum, the transfer chamber having a movable wall in the form of an elevatable stage that sealingly closes the transfer chamber in its outermost position. There is a small sealingly closed interface chamber extending between the movable walls when the cassette is mounted onto the machine. A vacuum pump evacuates the interface chamber in preparation for lowering of the cassette into the transfer chamber by an elevator mechanism.

This application is a continuation of application Ser. No. 07/972,659,filed Nov. 6, 1992 abandoned.

CROSS-REFERENCE

The present application is related to U.S. patent application Ser. No.07/973,112, filed on the same date as this application, entitled"MICRO-ENVIRONMENT LOAD LOCK," by Gordon P. Krueger (Docket No. 354),which is incorporated herein by this reference.

BACKGROUND

The present invention relates to systems for transporting materials,parts, or any other objects between controlled environments withoutintroducing contamination from an intervening uncontrolled environment,and more particularly, to such systems as applied to processes formanufacturing high density electronic integrated circuits.

A serious problem in integrated circuit manufacturing is the presence ofparticulates and other forms of contamination on and about the wafers onwhich the circuits are formed. For example, exposure to ambient airgives rise to wafer surface modification, such as by oxidation,nitrification, adsorption of water vapors or other impurities. Thissurface modification can require additional steps of wafer surfacepreparation prior to further processing. One of the major sources ofparticulate contamination is human-generated, including both particleswhich are released by human bodies and particles which are stirred up byequipment operators moving around inside a semiconductor processingfacility. This aspect of the problem has led to the development ofvarious forms of mechanized and automated processing systems, andenclosed carriers for transporting the wafers between and within suchsystems. However, the mechanisms themselves are potential generators ofparticulates. Thus the exclusion of particulates continues to be alimiting factor as device dimensions become smaller and smaller becauseof the need to avoid the presence of smaller and smaller particles, andbecause of increased demand for larger integrated circuits.

U.S. Pat. No. 4,995,430 to Bonora et al. discloses a sealable,transportable container that, in one configuration, provides aStandardized Mechanical Interface (SMIF) pod for semiconductor wafers,the pod or carrier including a box, a box door or panel that sealinglycloses the bottom of the box, and a wafer cassette that is supported onthe panel within the box. A processing station for receiving the carriercomprises a protective canopy having a loading port, a port door orplatform that closes the port being lowerable within the canopy togetherwith the panel and the cassette, the box being retained at the port. Thestation also includes a manipulator that can move the cassette from thelowered panel for processing of the wafers and then replace the cassetteon the panel, the panel being finally raised with the cassette to thebox, the reassembled carrier being released from the loading port. TheBonora et al. patent also discloses separate seals between the panel andthe box, between the box and the port, and between the platform and theport. A fluid injection/extractor that is mounted to the loading portand fluid-coupled through the box provides means for cleaning theinterior of the carrier by alternately evacuating and pressurizing thespace within the box. The Bonora et al. patent further discloses thatparticles that may have been on external surfaces of the panel and theplatform are trapped between the panel and the platform by thesimultaneous lowering of the panel with the platform.

Many contemporary wafer processing apparatuses are equipped with vacuumload locks. U.S. Pat. No. 5,044,871 to Davis et al. discloses a vacuumwafer carrier that is placed onto a vertically movable stage within anupper chamber of a load lock. When the stage is lowered, a cover of thecarrier remains supported by a floor portion of the upper chamber whilea cassette of the carrier is lowered into a lower chamber of the loadlock. The upper chamber is provided with a load lock cover that isclosed after the carrier is placed into the upper chamber. The upperchamber has a vacuum port and a purge port, and a vacuum seal isprovided between the floor of the upper chamber and the stage. In oneconfiguration, another vacuum seal is provided between the cover of thecarrier and the upper chamber floor. The carrier is represented asmaintaining a vacuum during transport and storage, the cover of thecarrier being secured by differential pressure.

Notwithstanding the disclosures of Bonara et al. and Davis et al.,typical systems for preventing contamination of sensitive articles byparticulates and the like at and between process stations remain subjectto one or more of the following disadvantages:

1. A relatively large volume must be evacuated prior to processing whenthe processing is to be performed at less than atmospheric pressure.This is true for the load lock of Davis et al. whether the upper lockchamber or the lower lock chamber is evacuated during a loading cycle.Similarly, Bonara et al. disclose evacuation of the carrier forcleansing the carrier by alternately evacuating and pressurizing thecarrier.

2. The lower lock chamber of Davis et al. is subject to contaminationfrom the upper lock chamber and from particulates that may fall onto thestage or that may stick to a bottom surface of the carrier. Similarly,the panel and platform of Bonara et al. are subject to this type ofcontamination. The contamination is not necessarily trapped between thepanel and the platform, but is likely to be dislodged and mobilized whenthe space between the carrier and the stage or platform is evacuated,the particles moving into an environment that is shared by the wafersduring processing.

Thus there is a need for a system that permits transport of wafers by acarrier between controlled environments without these disadvantages.

SUMMARY

The present invention meets this need by enabling the contents of afirst chamber to be moved into a second chamber without contaminatingeither chamber with ambient air.

In one aspect of the invention, a system for transporting an object froma first environment into a second environment without contamination byambient air comprises:

(a) a first housing having a first environment therein, the firsthousing having a movable wall for supporting the object and for closingthe first housing;

(b) a second housing having a second environment therein, the secondhousing having a movable wall for closing the second housing;

(c) means for locating the first housing relative to the second housingwith the movable wall of the first chamber in proximate contact with themovable wall of the second chamber, an interface volume extendingbetween the two movable walls;

(d) a passage for evacuating the interface volume, the passage beingisolated from the first and second chambers when the chambers are closedby their respective movable walls;

(e) means for moving the movable wall of the second housing into thesecond housing; and

(f) means for moving the movable wall of the first housing and theobject into the second housing.

Instead of exhausting the interface volume, it can be pressurizedthrough the passage if the chambers are to be pressurized.

Preferably the system includes means preventing the movable wall of thesecond housing from moving into the second housing until after theinterface volume has been evacuated.

The system can further include an interface seal for sealinglyconnecting the first housing to the second housing, the movable walls ofthe first and second housing having respective first and second wallseal means for sealingly closing the respective housings, the interfacevolume being bounded by the interface seal means and the first andsecond wall seal means when the housings are closed by the respectivemovable walls.

The interface seal means can include a first elastomeric ring seal onthe second housing for contacting an external surface of the secondhousing. The first wall seal means can include a second elastomeric ringseal on the movable wall of the first housing for contacting an internalsurface of the first housing. The third wall seal means can include athird elastomeric ring seal on the movable wall of the second housingfor contacting an internal surface of the second housing.

Preferably the interface volume is not more than about 1% of thecombined volumes of the first and second chambers for facilitatingefficient operation of the system.

The object can be a wafer cassette for holding a plurality of integratedcircuit wafers, the first housing can be a wafer carrier enclosure forholding a spaced plurality of wafers, and the second housing can be partof a semiconductor wafer processing machine.

The invention also includes a method of utilizing the system, whereinthe contents of the first chamber are moved into the second chamber bythe steps of:

(a) placing the movable wall of the first chamber in proximate contactwith the movable wall of the second chamber, with the interface volumeextending between the two movable walls;

(b) evacuating the interface volume or adding gas through the interfacevolume through the passage;

(c) moving the movable wall of the second chamber into the secondchamber; and

(d) moving the movable wall of the first chamber and the contents of thefirst chamber into the second chamber.

Generally the walls are moved simultaneously for preventing anycontamination that might remain between the movable walls fromcontaminating the second chamber, and to prevent generation ofcontamination. The two movable walls can be kept in contact during thesteps (c) and (d) of moving the walls, i.e., the walls are movedtogether. The movable wall of the second chamber can be horizontallydisposed, wherein the steps (c) and (d) of moving the walls compriseslowering the walls.

Preferably, when the object is to be subjected to reduced pressure inthe second chamber, the method includes the further steps of maintainingthe chambers in respective evacuated conditions. Thus the inventioneliminates the need for evacuating large volumes of air from thechambers while safely purging the facing surfaces and any volume betweenthe movable walls.

By maintaining the two chambers continuously in an evacuated condition,there is no requirement for evacuating a large volume of air inpreparation for a process step to be done under vacuum. Further, theinterface volume can be quickly evacuated to a destination separate fromeither chamber, thereby avoiding contamination of the chambers,particularly the second chamber, by contaminants that might otherwise bemobilized from between the movable walls during evacuation.

When the first chamber is initially at a superatmospheric orsubatmospheric pressure, the pressure in the second chamber iscontrolled to approximately match the initial first chamber pressure. Itis preferred that the method include the additional step of preventingmovement of the wall of the second chamber until the interface volumehas been evacuated or pressurized sufficiently to approximately equalizethe pressure of the interface volume with that of the second chamber.The step of preventing the movable walls from moving can be continueduntil the interface volume reaches approximately the initial firstchamber pressure.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a fragmentary, sectional, elevational view showing a portionof a wafer processing machine, the machine being equipped with anenvironmental interface system according to the present invention; and

FIG. 2 is a fragmentary, sectional, elevational, detail view withinregion 2 of FIG. 1.

DESCRIPTION

The present invention is directed to an environmental interface systemthat permits efficient storage and transfer of articles to be processed,while effectively excluding even very small particulates and othercontamination. Although the present invention is described primarilywith regard to its use for transferring articles under subatmosphericconditions, the invention is equally suitable for transferring objectsunder superatmospheric conditions.

With reference to FIGS. 1 and 2, a semiconductor wafer processingmachine 10 includes a base 12, a buffer chamber 14 being defined abovethe base 12 and having environmental control means 15 in fluidcommunication therewith for establishing a contaminant-free environmenttherein, typically under a partial vacuum that is suitable for waferprocessing as further described below.

The buffer chamber 14 is sealingly bounded by side walls 16 that extendupwardly from the base 12. The side walls support a top wall 20 of thechamber 14. A main chamber opening 18 is formed in the top wall 20. Anindexing elevator mechanism 22 that is supported by the base 12 includesa column member 24 and a stage member 26 that is fixably connected to anupper extremity of the column member 24. The column member 24 and thestage member 26 are vertically movable on an elevator axis 28. A columnbearing assembly 30 that is mounted below the base 12 laterally supportsthe column member 24. The column member 24 threadingly engages a screwdrive 32 that is fixably supported below the base 12 by conventionalmeans (not shown), the column member 24 being raised and loweredconventionally by the screw drive 32, which can include a stepper motor(not shown), the screw drive 32 being responsive to a controller 33.

The stage member 26 has an outwardly extending flange portion 34, theflange portion 34 having an elastomeric stage seal 36 that sealinglyengages the top wall 20 when the stage member 26 is fully raised by thescrew drive 32. Axially flexible bellows members 38 enclose that portionof the column member 24 that extends within the chamber 14. A wellmember 40 extends below the base 12 for supporting the bearing assembly30. The bellows members 38 are sealingly connected between the stagemember 26 and a bottom extremity of the well member 40.

Thus, the chamber 14 can be totally enclosed by a housing defined by thebase 12, the side wall 16, the top wall 20, and the stage member 26.Accordingly, a controlled environment, having a subatmosphere transferchamber pressure P_(T) that can be a suitable vacuum for processing ofsemiconductor circuit wafers, is maintainable in the buffer chamber 14by the environmental control means 15 when the stage member 26 sealinglyengages the top wall 20, so that the chamber 14 is sealingly isolatedfrom ambient air. The stage member 26, when in its raised position, asshown in FIG. 1, closes the main opening 18 in the top wall 26. Thus,the stage member 26 serves as a movable wall of the chamber 14.

A semiconductor circuit wafer carrier 42 is sealingly mountable on thetop wall 20 of the chamber 14, the carrier 42 having a housing formed byan enclosure portion 44, a top 45, and a bottom cover member 46. A wafercassette 48 is releasably located on the bottom cover member 46. Thecassette 48, having a latch assembly 50 for releasably anchoring thecassette 48 to the bottom cover member 46, holds a vertically spacedplurality (typically 25) of horizontally oriented semiconductor wafers52. The bottom cover member 46 of the carrier 42 holds an upwardlyfacing elastomeric cover seal 54 that engages a base member 56 of theenclosure portion 44. A sealed carrier chamber 58 is thereby providedwithin the carrier 42 when the bottom cover member 46 sealingly engagesthe base member 56 of the enclosure portion 44, the seal 54 beingeffective for maintaining a carrier pressure P_(C) that is at least apartial vacuum within the carrier 42. The stage member 26 carries aplatform latch 59 for insuring that the bottom cover member 46 remainsin place on the stage member 26 during movement thereof, the latch 59being activated following placement of the carrier 42 onto the machine10. The latch 59 is released in a conventional manner followingcompletion of a processing cycle of the machine 10, wherein the stagemember 26 is moved into the buffer chamber 14 as described below.

The top wall 20 of the machine has a downwardly facing elastomeric seal60 that engages a seal land 62, the land 62 being formed on theunderside of the base member 56. An outer perimeter extremity 64 of thebase member 56 extends slightly below the land 62 and within a slopingguide flange 66 that extends upwardly from the top wall 20. An interfacechamber 68 is formed between the bottom cover member 46 of the carrier42 and the stage member 26 and within the main chamber opening 18 whenthe stage member 26 is fully raised, the chamber 68 having an interfacepressure P_(I).

An important feature of the present invention is that the interfacechamber 68 is provided with an interface port 70. The interface port 70is in fluid communication with the interface chamber 68 and isfluid-connected to an interface pump means 72 for evacuating theinterface chamber 68 in preparation for lowering the stage member 26 bythe elevator mechanism 22. Following evacuation of the interface chamber68 by the pump means 72 to a desired pressure P_(S) that isapproximately equal to the carrier pressure P_(C), and with the transferchamber pressure P_(T) being also maintained approximately equal toP_(C), the cassette 48, together with the bottom cover member 46, islowered into the buffer chamber 14 on the stage member 26 by theelevator mechanism 22.

Another important feature of the present invention is that the wafercarrier 42, which provides a microenvironment for the wafers, need notbe placed in a separate housing to effect transfer. Thus, the carrier 42is exposed to ambient conditions during transfer, unlike the system ofDavis et al., U.S. Pat. No. 5,044,871 which requires an upper load lockchamber into which the carrier is placed. This results in lesscomplicated equipment, and quicker, more effective, and more efficienttransfer, since it is not necessary to evacuate the upper chamber of aload lock. By "ambient conditions," there is meant the environmentthrough which the wafer carrier is transported, which can be a cleanroom or other controlled or uncontrolled environment.

The interface chamber 68 has an interface volume (V_(I)) that is verymuch smaller than the chamber volume (V_(T)) of the buffer chamber 14and the carrier volume (V_(C)) of the carrier chamber 58, therebyfacilitating rapid evacuation of the chamber 58 for enhancing thethroughput of the machine 10. For example, the carrier volume V_(C) canbe on the order of 10 liters when the wafers 52 are 200 mm in diameter,and likewise the buffer chamber volume V_(T) is on the order of 10liters. Contrastingly, the interface volume generally is not more thanabout 100 cc. It can be only 40 cc based on an average depth of theinterface chamber 68 being 0.4 mm under the bottom cover member 46(within a diameter of 250mm) and an average depth of 5mm within adiameter of 260mm of the main chamber opening 18. Thus the interfacevolume V_(I) is preferably less than about 2% of the carrier volumeV_(C), less than about 2% of the buffer chamber volume V_(T), and lessthan about 1% of the combined volume of the buffer chamber 14 and thecarrier chamber 58. Grooves can be included in the interface to insurethat the interface is evacuated.

The interface port 70 (also referred to as a passage) preferably isfluid-connected to a pressure sensor 74, the sensor 74 providing asensor signal X to the controller 33 for preventing activation of thescrew drive 32 for lowering the stage member 26 from its closed positionuntil the interface pressure P_(I) has been lowered to approximatelymatch the carrier pressure P_(C) within the carrier chamber 58. For thispurpose, each of the wafer carriers 42 that is to be used with themachine 10 can be evacuated to a predetermined pressure such as 10⁻⁵Torr, the desired pressure P_(S) therefore also being approximately 10⁻⁵Torr. In this version of the present invention, the pressure sensor 77for the interface chamber 68 is operative for signalling the controller33 when the sensed interface pressure P_(I) drops to the predetermineddesired pressure P_(S). Correspondingly, the buffer chamber 14 ismaintained at approximately the same predetermined pressure by theenvironmental control means 15. It will be understood that when otherpressures are to be utilized within the buffer chamber 14, thecontroller 33 is operative for appropriately signalling the controlmeans 15 for restoring the predetermined pressure within the chamber 14,prior to operation of the elevator mechanism 22. Further, theenvironmental control means 15 also provides a feedback signal,designated environment signal Z, to the controller 33 for verifyingresponses by the control means 15.

Preferably, and as further shown in FIG. 1, the carrier 42 is providedwith a data block 76 for indicating the pressure within the carrierchamber 58 to data input means 78 on the machine 10, the data inputmeans 78 providing a data signal Y to the controller 33. In its simplestform, the data block 76 is merely a label on which the pressure P_(C) iswritten, perhaps together with wafer type identification and/or otherdesired process parameters for use in the processing by the machine 10.Other forms of the data block 10 that are within ordinary skill in theart of data communication include an electronic circuit havingnon-volatile memory, and movable mechanical indicia. Most preferably,and as indicated at 77 in FIG. 1, the data block 76 is responsive to theactual carrier pressure P_(C), being fluid connected to the carrierchamber 58 through the enclosure portion 44. Correspondingly, the datainput means 78 can comprise a conventional keypad, an electronicinterface, or a position transducer, each being within the ordinaryskill of those in data communication arts.

In preferred implementations of the present invention wherein the datablock 76 is operative for signalling the actual carrier pressure P_(C)to the controller 33 by way of the data input means 78 as describedabove, the environment signal Z also and correspondingly is indicativeof the transfer chamber pressure P_(T). Accordingly, the controller 33is implemented in any conventional manner for preventing activation ofthe screw drive 32 until the sensor signal X approximately matches theinterface signal Y while the environment signal Z is also beingmaintained in approximate correspondence with the interface signal Y.

As described above, the pressure, P_(T), of the buffer chamber 14 can becontrolled at suitable pressures that are compatible with the variouswafer processes that are provided by the machine 10, the wafers alwaysbeing subjected to the pressure P_(T) when the stage member 26 is notsealingly closing the main chamber opening 18. Alternatively, and asfurther shown in FIG. 1, the buffer chamber 14 is also sealinglyseparated by a divider wall 80 from a transfer chamber 82, a slit valve(not shown) being provided in the divider wall 80 for permitting serialtransport of the wafers 52 between the chambers 14 and 82 byconventional means (not shown), but pressureisolating the transferchamber 82 from the buffer chamber 14 at times when it is desired toevacuate the transfer chamber 82 to a hard vacuum such as 10⁻¹⁰ Torr.The chamber 82 can have a robot therein for transferring the wafers toprocesses such as deposition by chemical vapor deposition or plasmadeposition; cleansing and etching; and the like. Alternatively, thechamber 82 itself can be used for such processing.

As further shown in FIG. 1, an exemplary configuration of the presentinvention has the carrier 42 loaded onto the machine 10 from above, thecassette 48 being lowered into the buffer chamber 14 by operation of theelevator mechanism 22. Alternatively, the base 12 can be inverted, thecarrier 42 also being inverted and loaded from below the machine 10. Inanother alternative, the carrier 42 can be loaded at one side of themachine 10, the wafers 52 being oriented vertically rather thanhorizontally as shown in the drawings, the elevator mechanism 22operating horizontally rather than vertically.

In use of a system according to the present invention, the wafer carrier42 is placed on the top wall 20 of the buffer chamber 14 with the basemember 56 inwardly of the guide 66. The interface chamber 68 is thenevacuated with the pump 72 via the interface port 70. When the pressureP_(S) in the interface chamber 68 is about equal to the pressure P_(C)in the carrier pressure, the top wall 20 of the buffer chamber 14 islowered via the elevator mechanism 22, with the result that the bottomcover member 46 of the wafer carrier 42, as well as the wafer cassette48, are lowered into the buffer chamber 14, as shown in phantom inFIG. 1. The semiconductor wafers 52 can then be accessed for processing.

The lowering and access to the semiconductor wafers 52 can be effectedthree ways. First, the top wall 20 can be lowered stepwise, i.e., byindexing, with the top wall lowered to provide access to onesemiconductor wafer at a time. Secondly, the top wall 20 can be loweredall the way to the bottom of the buffer chamber 14, and left in thatposition so that all the semiconductor wafers are accessible. In a thirdversion, after the top wall 20 is lowered to the lowest position as inoption 2, the top wall is then raised stepwise, thereby providing accessto the semiconductor wafers, one at a time.

If the carrier 42 is under vacuum, then atmospheric pressure on thecarrier 42 exerts more force on the column member 24 than can be handledby a stepper motor. In such a situation, it is necessary to firstdisengage the stepper motor before loading the carrier 42 in place.After disengaging the stepper motor, the carrier 42 is positioned,latched carrier in place, and then the carrier 42 is lowered using adifferent motor. Next, the stepper motor is reengaged to stepwise raiseand/or lower the cassette 48.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, the environment maintained in the buffer chamber14 and the carrier chamber 58 need not be subatmospheric, but can be anynon-ambient environment, such as a low temperature environment; a hightemperature environment; special gaseous environments such as nitrogen,argon, oxygen, or the like atmospheres; high pressure environments; andcombinations thereof. All that is required under these conditions is tomodify the environment in the interface chamber 68 through the port 70.Also, the object in the carrier chamber 55 need not be a cassette 58,but can be any object. Moreover, if the object is a cassette 58, it neednot hold semiconductor wafers 52, but it can hold glass wafers or otherproducts.

Therefore, the spirit and scope of the appended claims should notnecessarily be limited to the description of the preferred versionscontained herein.

What is claimed is:
 1. An apparatus for transporting an object from afirst environment into a second environment without contamination byambient air, the system comprising:(a) a first housing having the firstenvironment therein, the first housing having a moveable wall forsupporting the object and for closing the first housing; (b) a secondhousing having the second environment therein, the second housing havinga moveable wall for closing the second housing; (c) a locator forlocating the first housing relative to the second housing with themoveable wall of the first housing in proximate contact with themoveable wall of the second housing, an interface volume extendingbetween the two moveable walls; (d) a passage in fluid communicationwith the interface volume to remove contaminants from said interfacevolume, the passage and said interface volume being isolated from thefirst and second chambers when the chambers are closed by theirrespective moveable walls; and (e) an elevator for moving the moveablewall of the second housing and the moveable wall of the first housinginto the second housing.
 2. The apparatus of claim 1 including anevacuator for evacuating gas from the interface volume through thepassage.
 3. The apparatus of claim 2, comprising means for preventingthe movable wall of the second housing from moving into the secondhousing until after the interface volume has been evacuated.
 4. Theapparatus of claim 2, wherein the object is a wafer cassette for holdinga plurality of integrated circuit wafers, the first housing is a wafercarrier enclosure, and the second housing is part of a semiconductorwafer processing machine.
 5. The apparatus of claim 1, furthercomprising an interface seal for sealingly connecting the first housingto the second housing.
 6. The apparatus of claim 5, wherein the movablewalls of the first and second housing have respective first and secondwall seals for sealingly closing the respective housings, and whereinthe interface volume is bounded by the interface seal and the first andsecond wall seals when the housings are closed by the respective movablewalls.
 7. The apparatus of claim 6, wherein the first wall sealcomprises an elastomeric ring seal on the movable wall of the firsthousing for contacting an internal surface of the first housing.
 8. Theapparatus of claim 7, wherein the second wall seal comprises anotherelastomeric ring seal on the movable wall of the second housing forcontacting an internal surface of the second housing.
 9. The apparatusof claim 5, wherein the interface seal comprises a first elastomericring seal on the second housing for contacting an external surface ofthe first housing.
 10. The apparatus of claim 1, wherein the interfacevolume is less than 1% of the combined volumes of the first and secondchambers.
 11. The apparatus of claim 1 wherein the second housing has atop, and the passage extends through the top to the interface volume.12. The apparatus of claim 1 wherein the locator is positioned so thatthe first housing is exposed to ambient conditions when the firsthousing is located so that the movable wall of the first housing is inproximate contact with the movable wall of the second housing.
 13. Theapparatus of claim 1, further including a pressure source incommunication with said interface volume through said passage.
 14. Theapparatus of claim 1, further including a vacuum source in communicationwith said interface volume through said passage.
 15. The apparatus ofclaim 1, further including a pressure source and a vacuum source incommunication with said interface volume through said passage.